Abstract:

Recent years has seen growing interest in biomedical applications of homogenous -Ti alloys, such as Gum MetalTM (i.e. Ti – 24(Nb+Ta+V)-(Zr,Hf)-O (at.%)) with non-toxic elements that exhibit a low Young’s modulus, high strength, superelasticity and excellent biocompatibility. Only a few key alloying elements such as Nb, Ta and Zr in -Ti alloys exhibit this multifunctional property, which is predicted by a d-electron theory based around a “magic” parameter of the electron/atom ratio e/a = 4.24. There is limited research literature reporting producing this quaternary alloy via the Powder Metallurgy (PM) process. In this study, two compositions based on quaternary system with e/a of 4.24, Ti-2Nb-22.5Ta-2Zr (at.%) i.e. TNT22Z, and Ti-23Nb-0.7Ta-2Zr (at.%) i.e. TNTZ aka Gum MetalTM were studied. The effect of e/a ratio on β phase stability and Young’s modulus via PM process, others alloys varying with Ta content of x = 5 - 22.5 in the Ti-2Nb-(x)Ta-2Zr (at.%) quaternary alloys were also studied. The production of fully dense homogenous β-Ti alloys based on these high temperature refractory alloying elements is challenging. As such, the focus was to study the novel multifunctional quaternary TNT22Z in comparison with TNTZ produced via PM process and primarily investigate the effect of e/a ratio on β phase stability and Young’s modulus. An e/a at 4.24 was of particular interest. The process involved two sintering cycle profiles to obtain a transient and persistent liquid phase sintering, with the aim of achieving an optimal densification and homogenous phase constituent with low Young’s modulus. In-situ Neutron Diffraction (ND) and ex-situ sintering studies validated that with an e/a value of 4.24 stabilized homogenous β phase was obtained a β in both TNTZ and TNT22Z samples. Three different sintering furnaces, one graphite (on TNTZ for as reference), a molybdenum (on TNTZ and TNT22Z comparison and other alloys like TNT15Z and TNT5Z) and the final with using niobium (as part of in-situ ND studies) heating elements conditions, were used in this study. A significant amount of impurity, TiCNO phase (~ 1-8 wt.%) was obtained in TNTZ sintered in the graphite furnace, and this seems to increase the α phase and as such, no homogenous β phase was obtained. Fully dense TNTZ of 99.50% with β phase and some remnant small amount of TiCNO (~ 0.78-1.91 wt.%) was obtained by sintering in molybdenum furnaces, while in the TNT22Z, a sintered density of 98.48% with homogenous β phase was obtained, but no impurities were found. The addition of Ta in Ti-2Nb-(x)Ta-2Zr (at.%) effectively varies the e/a values from 4.07 to 4.24 and reveals that increasing the e/a value significantly lowered the interdiffusion coefficients, impeded densification and homogeneity. Prolonging duration of the isothermal hold to 5 hrs at a prescribed temperature of 1600°C, overcame the long diffusion paths and allowed better homogeneity. In-situ neutron diffraction experiments offer an insight on the reaction pathways and phase transitions of the quaternary system, revealing the temperature range for the transformation of αTi→βTi (from 821.03°C - 1055.16°C = ~ 234°C) in TNTZ, while in the TNT22Z system, the transformation of αTi→βTi was completed at significantly narrower temperature range (i.e. 910.64°C - 974.5°C = ~ 63.86°C). TNT22Z and TNTZ, both with an e/a of 4.24 achieved a low Young’s modulus of 33.76 GPa and 9.41 GPa, respectively, as measured by nanoindentation testing. These sintered samples also exhibit a remarkable elastic recovery and shape memory effect behavior, caused by stress-induced phase transformations. Combining the results from the experiments provided several important conclusions. The powder metallurgy near-net-shape process showed a pathway to obtain near dense parts for homogenous β- Ti alloys; novel Ti-2Nb-22.5Ta-2Zr (at.%) and TNTZ quaternary alloys. The e/a at 4.24 proved a key indicator for producing stabilized β phase in both quaternary alloys. Additionally, Ti-2Nb- 22.5Ta-2Zr (at.%) and TNTZ exhibited low Young’s Modulus and multifunctional property behavior. Both TNT15Z (with e/a at 4.17) and TNT5Z (with e/a at 4.07) exhibited high Young’s Modulus and no shape memory effect behavior was observed.